I. Field of the Invention
This invention relates generally to wireless transmitters. More specifically, the invention relates to quadrature modulation of a transmission signal in a wireless transmitter.
II. Description of the Related Art
Modern digital communications systems often use quadrature techniques to impress data on a modulated signal. The use of quadrature techniques allows twice as much data to be transferred within the same bandwidth compared to single-phase modulation techniques. Quadrature techniques can be applied in many types of systems using a variety of modulation and access techniques. For example, quadrature techniques can be applied in code division multiple access (CDMA), time division multiple access (TDMA) and frequency division multiple access (FDMA) systems as well as others. Using quadrature techniques, at the transmitter, a quadrature modulator impresses a portion of the signal energy on an in-phase (I) channel and the remainder on a quadrature (Q) channel which is 90 degrees out of phase in comparison with the I channel. At the receiver, the energy from each channel can be separately recovered by a quadrature demodulator.
In many modern wireless communication systems, the cost of the remote communication unit is a substantial barrier to extensive deployment. For example, in a satellite based, wireless local loop telephone system, the cost of the handset to the end consumer can be a major factor in determining the penetration into the market which the wireless service will enjoy. For this reason, considerable effort to techniques has been expended in recent years revolutionize remote unit architectures so that they cost less.
In a typical wireless transmitter, a digital representation of a baseband signal is generated by digital circuitry. An analog to digital converter (A/D) is used to convert the digital representation to a baseband analog signal. The baseband signal is upconverted from baseband to a fixed intermediate frequency (IF) where a variety of signal processing functions are applied. For example, the gain of the signal can be set to accommodate current system conditions. The signal can be subjected to rigorous filtering in order to reduce transmission spurs. The conditioned IF signal is converted to a channel sensitive transmission radio frequency (RF). The RF signal is transmitted over the wireless link.
The main issues in designing a quadrature modulator are maintenance of quadrature phase between the I and Q channel, minimization of differential gain errors and differential direct current (D.C.) offsets between the I and Q channels as well as minimization of carrier leakage. Each of these factors can cause spurious output power to be generated which can interfere with the transmitted signal as well as signals in adjacent bands. In addition, each of these factors can introduce distortion into the transmitted signal which can result in an increased bit-error rate (BER) at the receiver and an undesired spreading of the signal spectrum. In practical circuit designs, these factors vary with temperature, device biasing, component aging and frequency, often making readjustment during operation necessary. In addition to baseband signal processing, IF signal processing can be used to accommodate for some of these errors.
In order to reduce the cost of the remote unit, the luxury of using IF techniques is no longer economically practical in many situations. Alternatively, modern communication transmitters have begun to use direct conversion techniques which provide for the direct conversion of a baseband signal to a radio frequency signal. Direct conversion systems are characterized by the use of a single up-conversion stage using a high-frequency quadrature modulator.
To construct a high frequency, direct conversion circuit, it is necessary to mount many extremely small devices on a single semiconductor chip substrate. These devices are closely packed together and, further, because of their size, are extremely sensitive to stray currents. Because of the dense packing and the associated network of interlaced wires, the circuits are susceptible to crosstalk between the various components. This crosstalk is primarily due to capacitive coupling between adjacent wires, but may also result from inductive coupling and transmission line effects. In addition, D.C. offsets and gains realized even on adjacent signal paths on a single substrate can vary significantly. Although through careful circuit design and layout these factors can be controlled, the unpredictability of extremely complex systems results in random variations in these factors from path to path, from part to part and from assembled system to assembled system.
Despite these design challenges, direct conversion architectures provide several desirable features. For example, direct conversion techniques tend to require less circuitry leading to a higher efficiency and reduced D.C. power requirements. Direct conversion transmitters are typically less costly to manufacture and smaller in size than conventional systems. FIG. 1A is a block diagram illustrating a basic direct conversion transmitter. As shown, a transmit modem (TM) 10 feeds a quadrature modulator (QM) 14 which in turn drives a power amplifier (PA) 16. The transmit modem 10 generates the complex baseband information signals. The quadrature modulator 14 provides direct conversion of the combined complex baseband information signals to the RF transmit frequency. The power amplifier 16 amplifies the RF signal for transmission over the wireless link.
Because a direct conversion system is very sensitive to quadrature modulator gain imbalance, phase imbalance and D.C. offsets, careful control over quadrature modulator errors is required. If quadrature modulator errors were more or less static, then, a simple calibration of the phase, gain, and offset errors would suffice. However, as noted above, the quadrature modulator errors can be expected to change with temperature, channel frequency, device biasing and component aging, and, thus, some means of "tracking" the quadrature modulator errors is needed.
Several error correction techniques have been studied in the literature such as Cartesian feedback, feedforward and predistortion. Using predistortion, the baseband signal is passed through a nonlinear system having an inverse characteristic compared to those of the quadrature modulator causing the overall system to be linear.
FIG. 1B is a block diagram illustrating a basic direct conversion transmitter which incorporates predistortion. In comparison with FIG. 1A, a predistorter 12 has been inserted between the transmit modem 10 and the quadrature modulator 14. Ideally, the predistorter 12 exhibits the inverse transfer function of the undesired characteristics of the quadrature modulator 14. The transmit modem 10 generates the complex baseband information signal which is then pre-corrected in the predistorter 12 to compensate for anticipated errors which will be introduced by the quadrature modulator 14.
Several approaches have been used to determine the value of the transfer function of the predistorter. In J. K. Cavers, "A Linearizing Predistorter with Fast Adaptation," IEEE Trans. Vehicular Technology, vol. 39, no. 4, pp. 374-382, November 1990, a look-up table having input power as the index and complex gain as the table entries is used. The table look-up method can be used to fit any gain profile, and performance can be adjusted by increasing or decreasing the table size. The second approach, as detailed in S. P. Stapleton, G. S. Kandola, and J. K. Cavers, "Stimulation and Analysis of an Adaptive Predistorter Utilizing a Complex Spectral Convolution," IEEE Trans. Vehicular Technology, vol. 41, no. 4, pp. 387-394, November 1992., uses two polynomial equations to fit the desired complex gain curves. This method is simpler to implement, but arbitrary gain profiles may be difficult to fit with polynomial functions. The polynomial method, because it has fewer variables, is also easier to initialize and update than the table look-up method.
A binary search is proposed by R. Datta and S. N. Crozier, "Direct Modulation at L-Band using a Quadrature Modulator with Feedback," International Mobile Satellite Conference, Ottawa (IMSC'95) pp. 383-388. The binary search is used to find the predistortion parameters by the measurement of energies at two or more frequencies during the calibration process. The binary search method is an iterative process which continues until the process converges on an acceptable value. The iterative nature of the binary search process can consume a significant amount of system resources and require significant amount of time. In addition, the binary search algorithm converges to the correct solution only if the correct solution is within the initial range of uncertainty.
Each of the prior art methods requires a substantial amount of processing power and can be exceedingly difficult to implement. In addition, they may each require a substantial amount of time to converge upon the optimal values and may not be robust under certain operating conditions.
Therefore, there has been a need in the industry, to develop a robust and reliable determination of predistortion parameters which does not consume an excessive amount of processing power.